This invention relates to improvements in the manufacture of board products from comminuted lignocellulosic materials and adhesive binders and, more particularly, to the manufacture of such boards having substantially greater flexural strength, stiffness, fastener-holding properties and resistance to splitting than has heretofore been obtainable.
At present, boards made from wood and other lignocellulosic particles with appropriate thermosetting, thermoplastic or, in some cases, two-polymer organic binders have been manufactured as panels or in other shapes for limited applications in which no substantial flexural strength requirements exist. The restriction of the use of wood particle board to such limited applications results from the fact that the tensile strength and stiffness of wood particle board is much less than that of lumber, and thus prevents a very poor resistance to flexural or bending stresses normally encountered in structural or framing members such as beams, decking or panels. No suitable remedy to the low tensile strength and stiffness of wood particle board has been discovered to date which can raise the tensile strength and elastic modulus (i.e. Young's modulus) sufficiently to enable the board to resist such flexural loading without rupture or undue deformation and at the same time retain the economy of the board necessary to enable its use as a practical lumber substitute in applications requiring flexural strength.
A further characteristic of present wood particle boards which discourages attempts to use them in applications necessitating high resistance to tensile stress is their questionable nail or screw holding capability under stress conditions. The lack of strong internal integrity of conventional particle board products may result in splitting around screws and nails or, at best, a limited ability to retain the nails or screws tightly throughout extended stress application. Such limited capability of conventional particle board to be fastened as a portion of a stressed structure, and remain securely fastened over an extended period of time, can be as much a disadvantage in such applications as the board's inherent lack of tensile strength.
While the use of certain reinforcing materials such as wire meshes, expanded metal and the like in wood particle board has been proposed from time to time as a solution to the foregoing problems, such schemes provide neither the tensile strength nor the fastening capability needed to render such boards acceptable as lumber substitutes in flexural stress applications.